If you do not have access to fresh seawater, Red Sea Salt can be used to reconstitute sea water and seems to provide good results with marine cyanobacteria Synechococcus. Alternatively you may order sea water from NCMA (if you can afford it...).

For all media

Leave the seawater (SW) to age for 2 months at room temperature;
Note : This"aging" step is critical (but akin to black magic) because during that period bacteria present in the sea water will degrade many organic molecules that are probably detrimental to algae. Our experince shows that medium made with 'fresh' seawater will in many cases prevent cells from growing.